Recently, a multi-band and a multi-system of a wireless communication device such as a mobile phone unit is developed. A single wireless communication device has a plurality of wireless communication devices. On the other hand, downsizing of the wireless communication device is continuously required. Therefore, reducing of the number of components used in a wireless communication device is examined. For example, reducing of the number of components by removing an interstage filter of a transmit line or a receive line is examined. However, in this case, improvement of attenuation characteristic of a filter or improvement of isolation characteristic between a transmit terminal and a receive terminal of a duplexer is required.
There is supposed a method of connecting an inductor in series with a parallel resonator in a case where a ladder type acoustic wave filter is used, as a method of improving the attenuation characteristic of a filter or the isolation characteristic of a duplexer. However, with the method, there is a problem that insertion loss may be increased. Japanese Patent Application Publication No. 2011-71911 (hereinafter referred to as Document 1) discloses a filter achieving high attenuation, high isolation and low loss. In the filter of Document 1, a phase converter is connected in parallel with a resonator connected to a filter portion extracting a signal having an arbitrary frequency band. The filter converts a phase of a signal input into the phase converter into another phase that is different from a phase at the resonator by 180 degrees. Japanese Patent Application Publications No. 2004-242280 and No. 2005-136658 and International Publication No. WO2009/025106 Pamphlet disclose a technology in which a part of a transmit signal is extracted into a line having a capacitor, another part of the transmit signal leaking to a receive side is canceled by the signal passing through the line, and isolation characteristic is improved.
However, it is difficult for the filter of Document 1 to achieve the attenuation characteristic, the improvement of the isolation characteristic and the low loss together with each other, because it is not easy for a phase converter to convert a phase into another phase that is different from a phase at the resonator by 180 degrees.